Various types of circuits for control of the ignition instant of small gasoline engines have been proposed. Once such circuit uses a microprocessor which requires operating power, the operating power being derived from a storage stage which is coupled to the primary winding of the magneto. Those half-waves which are used to provide ignition energy are also used to provide energy to operate the control system, typically the microprocessor (see, for example, German Patent Disclosure Document DE-OS No. 30 06 228). Half-waves which are used to provide ignition energy are additionally applied to the control input of the microprocessor and furnish a speed signal as well as a reference signal in order to calculate the optimum ignition instant. This type of circuit has the advantage that the speed and reference signals, respectively, which are applied to a control input of the microprocessor will have the same polarity as the supply voltage applied to the microprocessor. It has been found, however, that the voltage supply of the microprocessor can be impaired when the engine operates at idling speed since the half-waves derived from the magneto generator winding, until the ignition time instant, are essentially short-circuited by the closed breaker switch, typically a transistor, of the transistorized ignition circuit. Consequently, voltage supply for the microprocessor is not insured and, particularly at low speeds, must be increased, which may require additional circuitry, or increase of the idling speed of the ICE merely in order to supply sufficient operating voltage to the microprocessor or similar control system therein. Otherwise, accurate firing of the spark plug, at the desired instant, may not be controllable.